Systemic Lupus Erythematosus (SLE) is an autoimmune disease characterized by excess production of type I interferon (IFN) and inflammatory cytokines, as well as by a lack of tolerance to self-nucleic acids, especially double stranded DNA (dsDNA). Evidence suggests that both extra-cellular and intracellular DNA stimulates IFN production in SLE. Indeed, Dr. Pascual's group recently found that oxidized mitochondrial DNA (mtDNA) may be a relevant source of extra-cellular DNA in SLE. While this DNA can be internalized by pDCs to trigger endosomal TLR9 activation and production of IFN, whether the same mtDNA stimulates cytosolic sensors in a cell-autonomous manner is unknown. The 3' repair exonuclease 1, known as TREX1, is the main 3'-^5' exonuclease in mammalian cells and degrades both single-stranded DNA (ssDNA) and dsDNA. TREX1 plays a critical role in removing the store of endogenous DNA to avoid inappropriate sensing and IFN production. Mutations in TREX1 lacking DNA nuclease activity result in Aicardi-Goutieres syndrome and SLE. DNA that accumulates in TREX1 deficient cells signals through STING, but little is known about the cytosolic sensor(s) that respond to this accumulation and connect downstream with the STING pathway. Most nucleic acid cytosolic sensors use adaptors to activate the inflammasome response and MAVS or STING to activate an IFN response. Our studies indicate that DDX41 is the critical cytosolic sensor to recognize dsDNA and trigger the IFN host immune response mediated by STING. Other studies indicate that DDX41 as well as IFI16 play little role in sensing self-DNA. In an effort to identify novel sensors recognizing self-DNA, we isolated and characterized DNA-binding proteins in monocytes and found known as well as novel DNA sensors. Among the latter, the apurinic/ apyrimidinic endonuclease 1, APEX1, works downstream of TREX1 and uses the STING pathway to trigger IFN and inflammatory cytokine responses. Based on our findings, we hypothesize that APEX1 is a critical molecule to sense self DNA in autoimmune diseases such as SLE. Our specific aims will (1) establish the role of APEX1 as a key cytosolic sensor for self-DNA, including mtDNA, in SLE; (2) characterize the role of the APEX1-STING-TBK1 complex in SLE.